An image pickup device, such as a digital camera and the like, picks up an image of an object under different environments, for example, under illumination by a fluorescent lamp indoors. As a fluorescent lamp is directly driven by a commercial AC power source, its luminance is varied with a frequency which is double a frequency of the commercial AC power source. Such variation of the luminance of the fluorescent lamp produces a so-called “flicker effect” that flickers appear in an image signal obtained by an image pickup device.
Commercial AC power sources used in different countries or areas have different frequencies. For example, a frequency of a commercial AC power source in Japan is 50 Hz in East Japan and 60 Hz in West Japan. Therefore, if an image pickup device is used in different countries or areas where commercial AC power sources (hereinafter abbreviated as “AC power source”) having different frequencies are used, it is required to correct a flicker depending on power source frequencies of the different countries or areas.
FIGS. 4A to 4E are views for explaining comparison of luminance variation of a fluorescent lamp with a timing of exposure time; FIG. 4A showing a voltage waveform of an AC power source (60 Hz), FIG. 4B showing luminance variation of the fluorescent lamp driven by the AC power source (60 Hz), FIG. 4C showing exposure time of an image pickup device at a shutter speed of 1/60 seconds, FIG. 4D showing luminance variation of the fluorescent lamp driven by a AC power source (50 Hz), and FIG. 4E showing a voltage waveform of the AC power source (50 Hz). FIG. 5 is a view showing an example of display of a flicker.
As shown in FIG. 4A, when an AC power source frequency is 60 Hz, its period (Ta) is 1/60 seconds. As shown in FIG. 4B, when a fluorescent lamp is driven by the AC power source shown in FIG. 4A, its brightness is varied at a period (Tb=Ta/2= 1/120 seconds).
As shown in FIGS. 4B and 4C, if exposure time (Tc) is set to 1/60 seconds, 2 periods (2×Tb= 1/60 seconds) of luminance variation of the fluorescent lamp match up to the exposure time (Tc). Accordingly, when an image is picked up at the AC power source frequency of 60 Hz under illumination of the fluorescent lamp, there is no variation of amount of exposure at periods Tc1, Tc2 and Tc3. Accordingly, there occurs no flicker.
On the other hand, as shown in FIG. 4E, when an AC power source frequency is 50 Hz, its period (Te) is 1/50 seconds. As shown in FIG. 4D, when a fluorescent lamp is driven by the AC power source shown in FIG. 4E, its brightness is varied at a period (Td=Te/2= 1/100 seconds).
As shown in FIGS. 4D and 4C, if exposure time is set to 1/60 seconds, there is a deviation between a period of luminance variation of the fluorescent lamp and the exposure time (Tc). Accordingly, when an image is picked up at the AC power source frequency of 50 Hz under illumination of the fluorescent lamp, the amount of exposure is different in periods Tc1, Tc2 and Tc3.
In this manner, different amounts of exposure for the periods Tc1, Tc2 and cause variation in the picked image, for example, as shown in FIG. 5. Pictures P1, P2 and P3 shown in FIG. 5 are pictures picked in continuous exposure periods and have their respective stripes having different vertical positions.
In the example shown in FIGS. 4A to 4E, although there occurs no flicker when the image pickup device is used in an area where the AC power source is 60 Hz, there occurs a flicker when the image pickup device is used in an area where the AC power source is 50 Hz.
For the purpose of removing a flicker in either case where the AC power source frequency of the fluorescent lamp is 50 Hz or 60 Hz as described above, for example, Patent Document 1 discloses a television camera device including electronic shutter means that automatically varies exposure time depending on brightness of an object, fixing means that fixes a shutter speed of the electronic shutter means to a particular speed without depending on the brightness of the object in order to suppress a picture from being flickered, and means that optionally switches between automatically varying the shutter speed of the electronic shutter means and fixing the shutter speed to the particular speed.
[Patent Document 1] Japanese Patent No. 3422638